Cables are one method commonly used to convey electronic video signals from a source device (e.g., a video camera or a DVD player) to a destination device (e.g., a video display screen). Two types of cable commonly used for video transmission are coaxial cable and twisted pair cable. It is desirable for the video signal at the destination device to correspond accurately to the original video signal transmitted by the source device. “Insertion loss” is a term used to describe signal degradation that occurs when a video or other signal is transmitted over a transmission medium such as a cable. Insertion loss is typically caused by the physical characteristics of the transmission cable.
Typically, insertion loss is proportional to the cable length: longer length transmission cables will exhibit greater loss than shorter length cables. Coaxial cables typically exhibit less insertion loss than twisted pair cables. However, coaxial cables are more expensive and difficult to install than twisted pair cables. Twisted pair cables typically are manufactured as bundles of several twisted pairs. For example, a common form of twisted pair cable known as “Category 5” or “CAT5” cable comprises four separate twisted pairs encased in a single cable. CAT5 cable is typically terminated with an eight-pin RJ45 connector.
Video signals come in a variety of formats. Examples are Composite Video, S-Video, and YUV. Each format uses a color model for representing color information and a signal specification defining characteristics of the signals used to transmit the video information. For example, the “RGB” color model divides a color into red (R), green (G) and blue (B) components and transmits a separate signal for each color component.
In addition to color information, the video signal may also comprise horizontal and vertical sync information needed at the destination device to properly display the transmitted video signal. The horizontal and vertical sync signals may be carried over separate conductors from the video component signals. Alternatively, they may be added to one or more of the video signal components and transmitted along with those components.
For RGB video, several different formats exist for conveying horizontal and vertical sync information. These include RGBHV, RGBS, RGsB, and RsGsBs. In RGBHV, the horizontal and vertical sync signals are each carried on separate conductors. Thus, five conductors are used: one for each of the red component, the green component, the blue component, the horizontal sync signal, and the vertical sync signal. In RGBS, the horizontal and vertical sync signals are combined into a composite sync signal and sent on a single conductor. In RGsB, the composite sync signal is combined with the green component. This combination is possible because the sync signals comprise pulses that are sent during a blanking interval, when no video signals are present. In RsGsBs, the composite sync signal is combined with each of the red, green and blue components. Prior art devices exist for converting from one format of RGB to another. To reduce cabling requirements, for transmission of RGB video over anything other than short distances, a format in which the sync signals are combined with one or more of the color component signals are commonly used.
Thus, an RGB signal typically requires at least three separate cables for transmission of each of the red, green, and blue components and the combined horizontal and vertical sync information. If coaxial cable is used, three separate cables are required. If twisted pair conductors are used, three twisted pairs are also required, but a single CAT5 cable (which comprises four twisted pairs) can be used. Three of the four pairs may be used for the red, green, and blue components, respectively. The fourth pair is available for transmission of other signals (e.g., digital data, composite sync, and/or power). FIGS. 2 and 3 illustrate examples of how video signals may be allocated to the four pairs of twisted conductors in a CAT5 or similar cable.
In a CAT5 or similar cable, each end of each conductor is typically connected to one of eight pins of a standard male RJ-45 connector. In FIGS. 2 and 3, the first conductor pair corresponds to Pins 1 and 2; the second conductor pair corresponds to Pins 4 and 5; the third conductor pair corresponds to Pins 7 and 8; and the fourth conductor pair corresponds to Pins 3 and 6. For video signal configurations in which three or fewer conductor pairs are used for the transmission of the video signal, the remaining conductor pair or pairs (for example, the pair corresponding to Pins 3 and 6), may be used for communication of other signals, and/or for power transfer. Power transfer may be desirable if one of the devices is located remote from an external power source. For example, a source device may comprise a self powered laptop computer located at a distance from an external power source, such as a power outlet, while the destination device comprises a video projector display unit located in the ceiling of a room with a readily available AC power source. In such a configuration, the power needed to operate the transmitter may be conveyed from the receiver located near an AC power source via the twisted conductor pair not allocated for transmission of video signals. In such a configuration, the transmitter may be located within a wall or podium (e.g. in the vicinity of the laptop computer) without a nearby power source thus the transmitter can get its power from the receiver which is more likely to have a power source nearby.
FIG. 2 shows example pin configurations for a number of video signal formats. For example, with RGBHV video, as shown in the column headed “RGBHV” of FIG. 2, the twisted pair corresponding to Pins 1 and 2 carries the differential Red signals (i.e. Red+ and Red−) and the differential vertical sync signal (i.e. V Sync+ and V Sync−), the pair corresponding to Pins 4 and 5 carries the differential green signals (i.e. Green+ and Green−), and the pair corresponding to Pins 7 and 8 carries the differential Blue signals (i.e. Blue+ and Blue−) and the differential horizontal sync signal (i.e. H Sync+ and H Sync−). In FIG. 2, the conductor pair corresponding to pins 3 and 6 is allocated to carrying a digital signal and power.
For RGBS (i.e. RGB with one composite sync signal), in the example of FIG. 2, as shown in the column headed “RGBS,” the same pin assignments are used for the red, green and blue components as for RGBHV, with the composite sync signal combined with the Blue signal (i.e. Blue/C Sync+ and Blue/C Sync−). The composite sync signal could alternatively be combined with the Red component signal, or the Green component signal (as is done in the RGsB format, as shown in the column headed “RGsB” in FIG. 2). When the format to be transmitted is RsGsBs (i.e. composite sync signal added to each color component), as shown in the column headed “RsGsBs” in FIG. 2, the same pin assignments are used for each of the red, green and blue components as for RGBHV, except in this case the composite sync signal is added to each of the three color components.
In addition to showing example pin assignments for RGB signals, FIG. 2 also shows example pin assignments for component video, S-Video, and composite video. FIG. 3 shows an example of pin assignments that allow Composite video and S Video signals to share the same four-twisted pair cable.
Video standards require the regions known in the art as the front porch and back porch, i.e., the signal level before and after the horizontal synchronization pulse, to be at a DC ground level. However, it is common to find video sources with signals not referenced to ground. For instance, the video signal may be floating or biased above or below ground. Prior art systems use methods such as AC (e.g. capacitive) coupling to remove this undesirable DC bias.
However, AC coupling allows for a very large DC offset error on the input and distorts the video signal. The signal drifts up and down around it's bias point depending on video signal content. Also the video may be distorted (field tilt) due to the RC time constant inherent in such a circuit. To minimize the field tilt often large capacitor and resistor values are used which slow down the response time of the DC restore circuit. Therefore, capacitive coupling and other methods of prior art have drawbacks which degrade video quality and DC restore response time.